The one skincare product with randomized-trial evidence for fewer skin cancers and slower skin aging, if you apply enough of it every day.
Sunscreen absorbs (and to a small degree scatters) ultraviolet light before it damages skin cells. SPF measures protection against UVB, the burning rays; broad spectrum (US), the circled UVA logo (EU) or a PA+ to PA++++ rating (Asia) tells you it also covers UVA, which reaches deeper, passes through window glass and drives much of photoaging.
Filters come in two families. Mineral filters are zinc oxide and titanium dioxide. Organic ("chemical") filters include avobenzone, octocrylene, homosalate, octisalate, oxybenzone and newer ones such as bemotrizinol and bisoctrizole, which are sold in Europe and Asia. Both families work. The actives pages cover each filter; zinc oxide and avobenzone are the two most worth knowing.
In the Nambour randomized trial (1,621 Australian adults, 4.5 years), daily sunscreen cut the number of squamous-cell carcinomas by about 39% and slowed measurable skin aging; basal-cell carcinoma was not reduced. Long-term follow-up found half as many melanomas in the daily group, a borderline result based on small numbers. The trial tested SPF 15+ on the head, arms and hands in a sunny region, so the size of benefit elsewhere is extrapolated. FDA studies show several organic filters are absorbed into the blood; that triggers further safety testing but is not evidence of harm.
1,621 adults in Nambour, Australia, were randomized to daily SPF 15+ sunscreen or discretionary use for 4.5 years. The number of squamous-cell carcinomas was 39% lower with daily use (rate ratio 0.61); basal-cell carcinoma was not reduced.
Ten years after the Nambour trial ended, the daily-sunscreen group had 11 new melanomas versus 22 in the discretionary group (HR 0.50, borderline significance) and fewer invasive melanomas (3 vs 11).
In 903 Nambour participants under 55, daily broad-spectrum sunscreen users showed no detectable increase in skin aging over 4.5 years, and aging was 24% less than in the discretionary-use group, graded blind from skin casts.
In 48 healthy adults applying sunscreen at 2 mg/cm² to 75% of the body, avobenzone, oxybenzone, octocrylene, homosalate, octisalate and octinoxate all exceeded FDA's 0.5 ng/mL screening threshold after one application. The authors state this does not mean people should stop using sunscreen.